Re: How can you tell if a system is oscillating?

From: Mike Monett (mrmonett_at_yahoo.com)
Date: 09/15/04 

Date: 15 Sep 2004 02:27:37 -0700

see_my_signature_for_my_real_address@hotmail.com (Dr. David Kirkby)
wrote

[...]

> 2) The system, with a very high gain (over 100 dB) is acting as an
> oscillator. Since the gain is much higher between 5 and 85 MHz due to
> the filter, it will osciallate somewhere between there, where the loop
> gain is > 1. Whether or not this would result in a clean oscillation,
> rather than something that is noisy on a spectrum analyser, I don't
> know. I suspect it would, which makes me think it is just the
> amplified noise I am seeing, and no osciallations.
 
> If I put into the optical detector a light source which is RF
> modulated, the output RF modulation can be seen on the spectrum
> analyser if the bandwidth scanned is small (20 kHz or less), but can't
> be seen on the oscilloscope at all. It is probably burried beneth the
> noise.
>
> But how can I determine for sure if the system is oscillating or not?
> I don't think it is, but the question arrises how does one prove this
> one way or the other?
>
> Dr. David Kirkby BSc MSc PhD CEng MIEE

Sounds like you are looking at wideband noise with a center frequency
of 70MHz. If the system were oscillating, there is nothing to limit
the gain and you would see a square wave at the output instead of a
sine wave. So the sine wave is a good sign:)

When you add a small modulated signal and can see it on the spectrum
analyzer, it means the amplifier is linear. If it were oscillating, it
would be nonlinear and you would no longer be able to see this small
signal in the output.

I use a dental tool that has a short J-shaped piece of stainless steel
with a sharp point mounted in a plastic holder. This has ~1pF stray
capacity and acts as a small antenna. I use it to probe sensitive
inputs to see if they break into oscillation when I touch them, or if
a parasitic oscillation stops.

With the large gain in your system, I would expect touching an input
with a probe would cause the amplifier to break into oscillation, and
you would see a square wave at the output. A small rf signal at the
input would no longer be visible on a spectrum analyzer.

So my conclusion is the amplifier is operating as it should, and you
are to be congratulated on achieving such a high gain at these
frequencies without oscillation. It would be a good idea to check the
gain and phase vs frequency with a network analyzer to verify there is
no unwanted peaking in the response. But you have to be very careful
how you apply a signal to the input.

Most single-shield coax cable starts leaking around -60 to -80dB, so
the coax cables to the network analyzer can introduce unwanted
coupling from the input to the output. You can check for this by
grasping the cables at different points and see if the response
changes. Try double-shielded coax if there is any coupling.

Mike Monett